Solvation and ionization of hydroxyl groups in water–ice layers on silver (110)

Abstract

The solvation and ionization of hydroxyl groups on Ag(110) were examined with isotope exchange experiments involving OD and 18OD and temperature programmed desorption (TPD). Water adsorption onto oxygen-covered Ag(110) gives rise to the well known α, β, γ and δ peaks in TPD. The α-state represents multilayer water, the γ state an (OH)2·H2O complex, and the δ-state OH groups. Surface solvation of OH groups involves as many as 14±3 water molecules (β and γ) in the limit of zero OH coverage and decreases linearly to five water molecules for 0.15monolayer (ML) of OH. The solvating molecules are essentially all β-state molecules as the γ-state contains only 0.5 water molecules per OH group. The maximum coverage of solvating water molecules and OH groups is 1.04ML in good agreement with a perfect bilayer coverage of 1.18ML on Ag(110). From this we identify the β-state as an extended surface bilayer structure. Isotope exchange experiments demonstrated proton mobility in the OD/H2O adlayer at temperatures of 150K as well as migration of 18OD groups from the surface and into α-state, multilayer water molecules. This constitutes desorption of hydroxide ion into the water–ice multilayers and subsequent solvation.